1. Field of the Invention
The present invention pertains to TCAS and, more particularly, to a TCAS system for decoding ADS-B and/or TIS-B messages with improved receiver sensitivity.
2. Background
ADS-B: The Automatic Dependent Surveillance-Broadcast (ADS-B) is an avionics protocol that allows an ADS-B equipped system, such as an ADS-B equipped aircraft, to “see” on a display other ADS-B equipped systems in range of the subject system. An ADS-B equipped system may comprise an aircraft, a ground-based vehicle, such as a service vehicle at an airport, or anything else that one would want to “see” using ADS-B. Air traffic control may also use ADS-B to “see” ADS-B equipped systems.
Under the ADS-B protocol, an ADS-B equipped system periodically broadcasts its own state vector and other information without knowing which, if any, other ADS-B equipped systems might be receiving it, and without expectation of an acknowledgement or reply. ADS-B is “automatic” in the sense that no pilot or controller action is required for the information to be issued. ADS-B is “dependent surveillance” in the sense that the surveillance-type information so obtained depends on the suitable navigation and broadcast capability of the ADS-B equipped system making the transmission.
In operation, an aircraft or other ADS-B equipped system determines position information about itself, typically employing the global positioning system (GPS). The position information is employed to create a digital code, which may be combined with other information such as aircraft type, aircraft speed, aircraft flight number and whether the aircraft is turning, climbing or descending. The digital code, which may contain all of this information or in some cases more or less information, is updated several times a second and broadcast from the ADS-B-equipped system on a discrete frequency, called a data link. To transmit and receive ADS-B, an ADS-B equipped system may employ a Mode-S Extended Squitter (1090 ES) transponder, a Universal Access Transceiver (UAT), both a Mode-S Extended Squitter (1090 ES) transponder and a Universal Access Transceiver (UAT), or any equivalents thereof. ADS-B equipped systems, such as an aircraft or a ground station, within about 150 miles of an ADS-B transmission source may receive the ADS-B and display the received information. For example, a pilot in an aircraft cockpit can see traffic on a Cockpit Display of Traffic Information (CDTI). Additionally, air traffic controllers on the ground can see ADS-B traffic on their traffic display screen, as well as other radar targets.
RTCA: The Radio Technical Commission for Aeronautics or RTCA, Inc. is a private, not-for-profit corporation that develops consensus-based recommendations regarding communications, navigation, surveillance, and air traffic management (CNS/ATM) system issues. Its recommendations are used by the Federal Aviation Administration (FAA) as the basis for policy, program, and regulatory decisions and by the private sector as the basis for development, investment and other business decisions. RTCA publication DO-260A is entitled Minimum Operational Performance Standards for 1090 MHz Extended Squitter Automatic Dependent Surveillance-Broadcast (ADS-B) and Traffic Information Services-Broadcast (TIS-B). Appendix I of RTCA publication DO-260A (Appendix I) describes methods used to detect and correct data errors in ADS-B or TIS-B squitter messages.
TIS-B: TIS-B supplements ADS-B air-to-air services to provide complete situational awareness in the cockpit of all traffic known to the Air Traffic Control (ATC) system. TIS-B is a useful service for an ADS-B link in airspace where not all aircraft are transmitting ADS-B information. The ground ADS-B station transmits surveillance target information on the ADS-B data link for unequipped aircraft or aircraft transmitting only on another ADS-B link. TIS-B uplinks are derived from the best available ground surveillance sources, which may include ground radars for primary and secondary targets, multi-lateration systems for targets on the airport surface and/or ADS-B systems for targets equipped with a different ADS-B link.
ADS-B AND TIS-B MESSAGES: ADS-B and TIS-B messages are transmitted at 1090 MHz and consist of a data field of 112 bits that uses pulse position modulation (PPM) and is preceded by a four pulse preamble. The first 5 bits of the data field is the downlink field (DF). An ADS-B message has a DF field equal to 17 decimal (10001 binary) and a TIS-B message has a DF field equal to 18 decimal (10010 binary).
ERRONEOUS ADS-B AND TIS-B MESSAGES: Data error in an ADS-B or TIS-B message can be due to receiver noise for low level signals that are close to the receiver noise floor or due to overlapping signals that are generated by other transponder replies or from TIS-B ground stations, or other on-channel transmitters such as DME systems tuned to 1090 MHz. These overlapping signals (with the exception of DME systems) are termed “fruit signals” and can either be Air Traffic Control Radar Beacon System (ATCRBS) format of 15 bits or Mode S format of either 56 or 112 bits. Within the data field of 112 bits for an ADS-B or TIS-B message is a subfield of 24 bits that is used for parity encoding and is called the Parity/Identity field (PI field). The 24 bit PI field is generated by a polynomial division of the Mode S message by a fixed 24 bit polynomial. The PI field is used for detecting bit errors and may also be used for correcting bit errors in the message. Using error correction techniques set forth in Appendix I, it is possible to correct some messages which have erroneous bits due to receiver noise or fruit signals. In some cases, the messages can be corrected even if the overlapping fruit signals are much stronger than the ADS-B or TIS-B signal.
The error detection and correction methods set forth in Appendix I rely on the ability to correct bits that have obvious errors and mark other bits that may have less obvious errors as “low confidence” bits. After bits are either corrected or marked as “low confidence,” several well known algorithms can be used to correct the messages. These algorithms rely on the use of the PI field and the polynomial division operation to determine if the message has been corrected. These correction methods are set forth in Appendix I and include the “conservative technique,” the “brute-force technique” and the “whole message technique.” Using these algorithms, the undetected error rate is sufficiently small to meet system safety requirements.
In order to correct obvious bit errors and mark questionable bits as “low confidence,” the error correction algorithms set forth in Appendix I first determine the average RF level of the first 4 pulses in the message (these first 4 pulses are referred to as the “preamble”). The error correction algorithms set forth in Appendix I take a number of samples from the top of each of the 4 preamble pulses at a sample rate of 8 or 10 MHz and perform a number of tests and operations on the samples. Since it is possible to have overlapping higher amplitude fruit signals in the preamble, the algorithms attempt to remove these samples since they would provide an undesirable bias measurement of the RF level. This operation forms what is referred to in Appendix I as a “preamble reference level,” which is used for decoding the data in the ADS-B message.
Once the preamble reference level is determined, each of the data bits in the ADS-B message are processed using this level to determine the bit value (1 or 0) and whether it is a high or low confidence bit. Several error correction methods are described in the Appendix I, which are termed “multi-sample” due to the fact that each bit (1 micro-second in duration) has a number of samples which are processed (8 for an 8 MHz clock). The multi-sample error correction methods described in Appendix I include the “baseline multi-sample” and the “table look-up multi-sample.” Each of these methods rely on the determination of the preamble reference level. Other methods for data detection are possible, and other sample rates which are not defined in Appendix I may be used. Each of these methods assigns a bit value to each bit (1 or 0) and a confidence level for each bit (high or low). According to Appendix I, where obvious bit errors are detectable, the bit value is corrected by one of the multi-sample techniques, and the bit is labeled “high confidence.” This bit correction occurs prior to the error detection and correction algorithms described below.
After the message data bit value and confidence has been determined for the 112 bit message by one of the multi-sample techniques, an error syndrome, which is described in DO-260A and DO-185A (both of which are RTCA publications incorporated herein by reference), will be computed to determine if errors exist in the message. The error syndrome uses the PI field and a polynomial division operation in order to make the determination. If the error syndrome is non-zero, then several methods can be used to attempt to correct the message. The methods described in Appendix I are the “conservative technique,” the “brute-force technique,” and the “whole message technique.” Each of these methods makes use of the bit confidence level in order to attempt the correction.
The “conservative technique” can correct messages that have 12 or less low confidence bits that span no more than 24 Mode S bits and have no more than 7 consecutive low confidence bits. If this criteria is not met, then the “conservative technique” cannot be used to correct the message, since it may result in an unacceptable undetected error rate.
The “brute force technique” can correct messages that have 5 or less low confidence bits, regardless of where they occur in the message. If more than 5 low confidence bits exist, the message cannot be corrected using “brute force technique” due to the possibility of an unacceptable undetected error rate.
The “whole message technique” also uses low confidence bits and has similar limitations.
In the case of the error correction methods described in Appendix I (the “conservative technique,” the “brute-force technique” and the “whole message technique”), if too many low confidence bits exist or if bits that have errors are not marked as low confidence, the message cannot be corrected. The preamble reference level has a major effect on this determination. Some of the errors in the preamble reference level determination include errors due to noise for low level signals or errors due to overlapping fruit (interference) that corrupt the preamble reference level determination. If the preamble reference level determination is off, then the following undesirable situations may occur:                Bits that should be marked as low confidence bits may be marked as high confidence bits. If the bits have the wrong bit value, the error correction techniques cannot correct the message. The techniques will not attempt to correct bits that are marked as high confidence bits.        Bits that should be marked as high confidence bits may be marked as low confidence bits. If too many low confidence bits exist, the message will not be corrected.        
Another problem can occur when the timing of the preamble is not determined correctly (start of the Mode S data). The first bit of the Mode S data is 8 micro-seconds from the first preamble pulse. If the preamble pulse timing is not determined correctly, the data decoding may not be correct. The preamble timing determination may be in error due to noise for low level signals or errors due to overlapping fruit (interference). Some of the same issues will exist in the bit value and bit confidence determination, as described above.
Another problem may occur due to limitations of the decoding algorithms, namely, the enhanced preamble detection algorithms specified in section 4.1 of Appendix I or the enhanced bit and confidence declaration in section 4.2 of Appendix I. These algorithms, as set forth Appendix I, work well when the signal to noise ratio of the ADS-B/TIS-B signal is fairly good, e.g., when the signal level exceeds the noise level by at least 6 dB. The algorithms assume that if higher sensitivity is required, such as is the case for having A3 receiver sensitivity (as used herein “A3 receiver sensitivity” means Class A3 receiver sensitivity, as set forth in DO-260A), an active antenna will be used to improve the signal to noise ratio for the ADS-B receiver. However, when the ADS-B or TIS-B signals have a poor signal to noise ratio, such as when a non-active or passive antenna is used as part of a TCAS system, the Appendix I algorithms do not provide adequate performance.
Thus, a need exists for improved systems and methods for decoding ADS-B and TIS-B messages to achieve at least A3 receiver sensitivity while employing a passive antenna to receive the messages.